A switching power-supply device configured to control an output voltage by a switching operation using a switching element such as a transistor has a switching element serially connected to an input power supply and a primary winding of a transformer, and a switching control circuit configured to perform on-and-off control of the switching element for switching operation, and is configured to rectify and smooth a secondary electromotive force generated at a secondary winding of the transformer and to output a direct current voltage.
A synchronous rectification method of using an N-type transistor, instead of a diode, as an element configured to rectify the secondary electromotive force, and turning on-and-off the transistor in synchronization with a predetermined clock for rectification has been known.